1. Technical Field
This invention relates to device fabrication processes and in particular to device fabrication processes including the production of a silicon nitride region.
2. Art Background
Silicon nitride regions are used for many applications in the fabrication of electronic devices. For example, after the device, e.g., a semiconductor device, is fabricated, it is encapsulated in a silicon nitride material. This material is employed to prevent impurities in the ambient from reaching the device's active layers and, thus, from degrading device performance. Generally plasma deposition, a low-temperature process, is employed to avoid adversely affecting temperature-sensitive structures formed in previous processing steps. However, silicon nitride layers produced by plasma deposition can introduce other difficulties. If a high density of interface states, e.g., bulk imperfections or film instability, is produced upon deposition of the silicon nitride, device degradation rather than preservation occurs. (The density of defect states is advantageously monitored by measurement of, for example, capacitance-voltage (C-V) threshold shifts or changes in relative magnitude of breakdown voltages. These measurements are described in R. C. Sun and J. T. Clemens, "Effects of Silicon Nitride Encapsulation on MOS Device Stability", Proceedings International Reliability Physics Symposium, 1980, p. 244, IEEE80CH-1531-3. Film stability is measured by observing the threshold shift after accelerating thermal aging in the presence, if any, of hot carrier electrons. See, for example, W. G. Meyer and R. B. Fair, IEEE Transactions: Electron Devices, ED-30, 96 (1983).)
Detrimental effects on the layers underlying silicon nitride have been attributed to the presence of silicon-hydrogen bonds in the silicon nitride material. Some of these silicon-hydrogen bonds are relatively weak and tend to dissociate with subsequent migration of hydrogen through the silicon nitride material. The hydrogen produced and/or the free electrons on the associated silicon atom both contribute to trap states.
Thus, there has been a strong incentive to produce silicon nitride having a low hydrogen content and/or having strongly bonded hydrogen that is not mobile. This incentive has, in turn, yielded a concomitant desire for plasma processes which employ gases that are essentially free of hydrogen, or plasma processes that are designed to reduce or stabilize hydrogen in the silicon nitride. For example, Fujita, et al, have described the plasma discharge production of silicon nitride in Japanese Journal of Applied Physics, 23, L 144 (1984), Japanese Journal of Applied Physics, 23, L 268 (1984), and Journal of Applied Physics, 57, 426 (1985). This work employed (1) SiF.sub.4, nitrogen, and hydrogen, or (2) SiF.sub.2, nitrogen, and optionally hydrogen. The presence of extremely low levels of Si-H bonds, was inferred from the undetectable infrared absorption due to Si-H near 2200 cm.sup.-1. This low level was purposely sought to reduce traps. However, the bulk of the material contained a substantial concentration of oxygen that indicates the chemical instability of the film upon exposure to ambient air. Additionally, the deposition rate was so low, e.g., 100 .ANG./min for films that result in N/Si stoichiometries of approximately 1, that the process was not desirable for device fabrication. Thus, attempts to produce a high quality silicon nitride material by plasma deposition in practice have not been particularly successful.
Other attempts to produce silicon nitride containing a relatively low concentration of hydrogen have been equally futile. As described by Furukawa in Japanese Journal of Applied Physics, 23, 376 (1984), the use of NF.sub.3 with SiH.sub.4 /NH.sub.3 mixtures in thermal CVD led to the etching of the silicon substrate and did not result in deposition.